Upload wasm build as artifact

[avivace]

Just a quick change to expose the results of the wasm build in the CI as artifact so the js and wasm files can be easily downloaded

I can also do this for every build target, let me know how this looks @binji :smile:

[binji]

Thanks for the PR! Would you mind putting this in build_release.yml instead? That way it is only generated for releases rather than every build.